The present invention is directed to an improved process of shape selective conversion of hydrocarbons to aromatic compounds over a modified catalytic molecular sieve.
The term "shape-selective catalysis" describes unexpected catalytic selectivities in zeolites. The principles behind shape selective catalysis have been reviewed extensively, e.g., by Chen et al , "Shape Selective Catalysis in Industrial Applications," 36, Marcel Dekker, Inc. (1989). Within a zeolite pore, hydrocarbon conversion reactions such as paraffin isomerization, olefin skeletal or double bond isomerization, oligomerization and aromatic disproportionation, alkylation or transalkylation reactions are governed by constraints imposed by the channel size. Several principal shape selectivity constraints operate in zeolite catalyzed reactions. Reactant selectivity occurs when a fraction of the feedstock is too large to enter the zeolite pores to react. Product selectivity, on the other hand, occurs when some of the products of the reaction cannot leave the zeolite channels. Product distributions can also be altered by transition state selectivity in which certain reactions cannot occur because the reaction transition state is too large to form within the zeolite pores or cages. Another type of selectivity results from configurational constraints on diffusion where the dimensions of the molecule approach that of the zeolite pore system. A small change in the dimensions of the molecule or the zeolite pore can result in large diffusion changes leading to different product distributions. This type of shape selective catalysis is demonstrated, for example, in selective hydrocarbon aromatizations.
It is well known to convert saturated hydrocarbons to aromatic compounds by means of zeolite catalysts. Such a process is described in U.S. Pat. No. 3,756,942. The catalyst disclosed in this patent for producing aromatic compounds in high yield is a ZSM-5 type zeolite. These catalysts are described as suitable for selectively promoting the conversion of hydrocarbons such as paraffins, olefins, and naphthenes to aromatic compounds. Yields of 30 wt.% and more of aromatic compounds based on the non-aromatic portion of the hydrocarbon feedstock were reported in this patent.
Another such process is described in U.S. Pat. No. 3,760,024. This patent describes processes converting C.sub.2 -C.sub.4 paraffins and/or olefins to C.sub.6 -C.sub.10 aromatics over ZSM-5 type catalysts.
Another process is described in U.S. Pat. No. 4,180,689. This process involves a gallium-based HZSM-5 catalyst for conversion of light hydrocarbons into benzene, toluene and xylene (BTX) and hydrogen. This process, however, demonstrates little if any enhanced shape selectivity, producing xylenes generally at or near equilibrium, with the para-isomer around 24% of total xylenes. In addition, this process produces significant quantities of high-boiling C.sub.9 + aromatics, amounting to greater than 8% of the total amount of aromatics formed.
Other processes are described in U.S. Pat. Nos. 4,590,321 and 3,845,150. U.S. Pat. No. 4,590,321 describes conversions of non-aromatic compounds such as smaller paraffins and/or olefins to aromatic compounds over ZSM-5 type catalysts modified with phosphorus oxide. U.S. Pat. No. 3,845,150 describes processes for heat balancing in conversions of mixtures of paraffins and olefins to aromatics over ZSM-5 type catalysts. Other such processes are disclosed in U.S. Pat. Nos. 5,019,263, 4,326,994, and 4,117,026. Other types of processes are disclosed in Chen et al., Ind. Eng, Chem, Proc. Des. Dev., 25, 151-155 (1986), and U.S. Pat. Nos. 4,175,057 and 4,180,689.
Various methods are known in the art for increasing the para-selectivity of zeolite catalysts. One such method is to modify the catalyst by treatment with a "selectivating agent". For example, U.S. Pat. Nos. 5,173,461, 4,950,835, 4,927,979, 4,465,886, 4,477,583, 4,379,761, 4,145,315, 4,127,616, 4,100,215, 4,090,981, 4,060,568 and 3,698,157 disclose specific methods for contacting a catalyst with a selectivating agent containing silicon ("silicon compound").
Other types of selectivating agents are known. For example, U.S. Pat. No. 4,548,914 describes another modification method involving impregnating catalysts with oxides that are difficult to reduce, such as those of magnesium, calcium, and/or phosphorus, followed by treatment with water vapor to improve para-selectivity. European Patent No. 296,582 describes the modification of aluminosilicate catalysts by impregnating such catalysts with phosphorus-containing compounds and further modifying these catalysts by incorporating metals such as manganese, cobalt, silicon and Group IIA elements. The patent also describes the modification of zeolites with silicon compounds. U.S. Pat. No. 4,950,321 also describes modification of catalysts with phosphorus compounds.
Traditionally, ex situ selectivation of zeolites has involved single applications of the selectivating agent. It may be noted, however, that a suggestion of multiple treatments was made in U.S. Pat. No. 4,283,306 to Herkes. The Herkes patent discloses an attempt to promote crystalline silica catalyst by application of an amorphous silica such as ethylorthosilicate. The Herkes disclosure shows that a twice-treated catalyst is less selective than a once-treated catalyst, as measured by methylation of toluene by methanol, indicating that multiple ex situ selectivation confers no benefit and in fact reduces a catalyst's efficacy in this reaction.
Various organic compounds have been employed as carriers for selectivating agents in the impregnation methods applied to zeolite catalysts. For example, U.S. Pat. Nos. 4,145,315, 4,127,616, 4,090,981 and 4,060,568 describe the use of inter alia C.sub.5 -C.sub.7 alkanes as solvents for impregnation of zeolites with selectivating agents containing silicon. There has been no suggestion, however, of the use of lower volatility alkanes as carriers for impregnation of zeolites.
Therefore, it would be a significant advance in the art to overcome the above-described difficulties, disadvantages and deficiencies associated with conventional shape selective hydrocarbon aromatization processes in a manner that would enable use of modified catalytic sieves produced by methods which are both more efficient and safer and that would also increase product yields and reduce the proportion of undesirable impurities in the product of such aromatization processes.
The present invention solves the difficulties, disadvantages, and deficiencies inherent in the prior art by providing an improved process for shape selective hydrocarbon aromatization. The process of the invention produces higher rates of conversion of hydrocarbons to aromatic compounds, greater product selectivity, and greater product purity than do conventional processes.
Accordingly, it is a purpose of the invention to provide an improved process for the shape selective conversion of hydrocarbon compounds to aromatic compounds.
It is a further purpose of the invention to provide an improved process for converting hydrocarbons to aromatic compounds which overcomes the above-described difficulties, disadvantages, and deficiencies of the prior art practice of these processes.
Other purposes and advantages of the present invention will be more fully apparent from the following detailed disclosure and appended claims.